The present invention relates to a paper curl correction apparatus, and particularly to a paper curl correction apparatus for correcting curl of paper used in an electrostatic recording printing machine.
An important point in printing with an electrostatic recording system is close adherence between printing paper and a photosensitive body on which toner adheres to form a latent image. Proposed methods of transferring toner onto paper have included a corotron system using static electricity, a method of using belts or rollers and similarly using static electricity to press paper against a photosensitive body, and so on.
However, the latter method requires pressing apparatus and apparatus for cleaning residual toner after transfer. Therefore, the equipment using this method is expensive.
On the other hand, in the case of the former method, close adhesive force between a photosensitive body and printing paper depends on only electrostatic force. In the case where paper curls in a direction opposite to the curvature of the photosensitive body, the close adherence of the paper against the photosensitive body is so poor that toner cannot be transferred well. That is, a transfer failure or toner omission often occurs. Much toner is omitted, particularly at front and rear end portions of the paper with respect to the paper advancing direction.
The curl of paper to be subjected to printing may be caused not only by changes in environmental temperature and humidity but also by thermal influence after toner transfer before a fixing process. That is, because of thermal influence after toner transfer, paper becomes unable to keep its moisture equilibrium, so that the paper curls. Conventionally, to prevent transfer failure because of poor close adherence between a photosensitive body and paper because of curl of the paper and further because of imperfect toner fixing, sheets of paper which hardly curl have been used. Alternatively, a limit has been set in a printing range of paper so that, specifically, printing is not performed in the vicinity of front and rear end portions of the paper with respect to the paper advancing direction.
According to the former countermeasure, however, the kind of paper to be subject to printing is limited. As a result, the countermeasure does not afford wide use of the equipment. According to the latter countermeasure, on the other hand, printing cannot be performed effectively on the surface portion of paper, so that not only is there uneconomical consumption of paper per unit quantity of printing, but also, when paper curls on the way of conveyance, a floating paper edge of the curled paper is apt to be caught by any of a number of parts for conveying paper, so that stable paper running often is disturbed.